1. Field of the Invention
The present invention relates to an image forming apparatus employed in a copier, a printer and the like, and more particularly, to an image forming apparatus which visualizes an electrostatic latent image with a developer.
2. Description of the Related Art
In image forming apparatuses such as a printer and a copier utilizing an electrophotographic technology, an image forming unit forms an electrostatic latent image by a well-known electrophotographic process on the surface of an image holder having a drum- or belt-type organic photoconductor, and forms a toner image by developing the electrostatic latent image with a developer. Then, the toner image is transferred to a recording sheet directly or via an intermediate transfer medium, and the toner image is heat-fixed to the sheet with, e.g., a fixing device. In this manner, image formation is performed.
On the other hand, in recent years, high quality image formation has been promoted in the image forming apparatuses and image quality in printouts has been further improved. Especially, in on-demand type printing, it is necessary to output a job including various images at a high speed. For this purpose, the image forming unit of the image forming apparatus, especially the developer, is required to keep in an image-formation standby status. When the developer is continuously driven for an image having a low image concentration, toner staying for a long time is stirred with magnetic carrier excessively, thereby a material previously coated on the toner surface changes, and an image quality defect such as transfer failure or fogging occurs. To address these problems, the applicant has proposed a technique for suppressing degradation of a developing material upon occurrence of continuous low-resolution image jobs by forming a toner discharge patch in an inter-image (a portion between images where no image is formed).
As described above, when the developer is continuously driven for a low-concentration image, the low frequency of toner change may degrade the developing material and cause toner pseudo coagulation. Particularly in recent years, the toner diameter is being reduced to improve the graininess in image quality, and the adoption of small-diameter toner having diameters of 6 μm and even 4 to 5 μm is under review. In such small-diameter toner, the space between toner particles is smaller in comparison with large-diameter toner, and the tendency of toner coagulation is extremely high. In a case where the toner particles coagulate in a cluster, a print image, where the toner has been developed, transferred and fixed onto a recording sheet, has a white spot defect. The white spot defect becomes a serious problem in printing of photographic images which particularly requires high image quality.
The technique presented above is resultful to a certain degree. However, as the range of inter-image is narrowed too much in accordance with increased demand for improvement in productivity, it is difficult to ensure a large area for patch image formation in the inter-image. As a result, in some cases, forcible toner discharge cannot be sufficiently performed by using the patch in the inter-image. Further, in a case where a toner patch image is formed on a transfer belt as an application of the technique described above, it may be necessary to provide an upper limit to a discharge image density due to limitation of cleaning of the transfer belt. In such a case, it is conceivable that the function to discharge toner pseudo coagulation in developing is insufficiently performed. Accordingly, a further improved technique is required.